Practical_Antenna_Handbook_0071639586

434 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
three or four radials cut to l/4 at the design frequency of the vertical and spaced equally
around the compass (120 or 90 degrees apart, respectively). The radials should slope
down from the base at about a 45-degree angle. Some experimentation with their exact
length and slope may be in order. In contrast to radials laid directly on the earth’s surface,
elevated radials are resonant elements, and their length will have a strong effect on
both the SWR and the radiation efficiency of the vertical.
Yagi Antennas
The Yagi beam antenna is a highly directional gain antenna and is used in both HF and
VHF/UHF systems. The antenna can be easier to build at VHF/UHF than at HF. The
basic Yagi was covered in Chap. 12, so we will only show examples of practical VHF
devices.
Six-Meter Yagi
A four-element Yagi for 6 m is shown in Fig. 19.7. The reflector and directors can be
mounted directly to a metallic boom, but the choice of driven element and its feed system
require that the driven element be insulated from the boom.
The driven element shown in Fig. 19.7 is a folded dipole; because both conductors
are the same diameter, it provides a 4:1 step-up in feedpoint impedance relative to a
conventional dipole that is largely independent of the ratio of conductor spacing to
conductor diameter. Use of a folded dipole for the driven element is common practice
2.62 m
Director
No. 2
144.8 cm
2.67 m
Director
No. 1
116.9 cm
2.79 m
4:1
VHF balun
Driven
element
116.8 cm
Coax to XMTR
2.95 m
Reflector
Figure 19.7 Six-meter beam antenna.